1. Field of the Invention
The present invention relates to a pulse discharge laser with a passive element to prevent damaging arcs between laser electrodes, and particularly relates to a pulse discharge laser having a passive electrode arc protector to prevent arcing between laser electrodes due to improper preionization of a high pressure gas between the laser electrodes.
2. Description of the Related Art
FIG. 1 illustrates a prior art pulse discharge laser having a high pressure gas within a chamber 110. Electrodes 120 and 130 are contained within the chamber 110. X-rays are radiated into the chamber 110 to provide preionization energy to preionize the high pressure gas in the chamber 110. The x-rays are emitted from an x-ray generator 140 disposed adjacent to the chamber 110. A pulse generator 150 and a delay circuit 160 command the x-ray generator 140 to generate the x-rays after a power supply 170 is commanded to begin applying power to a water line capacitor 180. The water line capacitor 180 increases its charge voltage by the energy applied from the power supply 170 while the gas in the chamber 110 becomes preionized by the x-rays emitted from the x-ray generator 140. Preionization refers to the creation of a uniform electron density of at least 10.sup.6 electrons per centimeter cubed (cm.sup.3) in the discharge region prior to the application of the main discharge used for pumping the laser. When the voltage on the water line capacitor 180 reaches a predetermined voltage, the charge on the water line capacitor 180 passes through a rail gap switch 190 and appears across the electrodes 120 and 130. After the voltage on the water line capacitor 180 is applied to the electrodes 120 and 130, discharge occurs between the electrodes. Assuming the high pressure gas in the chamber 110 is properly preionized by the x- rays emitted from the x-ray generator 140, the voltage on the electrodes 120 and 130 uniformly discharges through the preionized gas between an entirety of the surfaces of electrodes 120 and 130, thus causing pulsed lasing to occur in the high pressure gas.
However, the high pressure gas in the chamber 110 is not always properly preionized when the voltage is applied across the electrodes 120 and 130. Sometimes proper preionization will not occur because of a timing error in the control signals received by the x-ray generator 140 and the power supply 170. Other times the high pressure gas will not be properly preionized when the x-ray generator 140 fails or has an intermittent delay. When proper preionization does not occur, the voltage arcs across the electrodes 120 and 130 and can cause severe damage to the laser. These arcs are particularly damaging inside of the chamber 110 where they can damage the surfaces of the electrodes 120 and 130. The metal electrodes 120 and 130 are typically plated with a second metal, and halides of the metal surface can form as a result of the arcing. These halides form a suspended dust in the optical path and deposit on window openings in the chamber 110. Arcing can also pit the plating on the electrodes 120 and 130, exposing the metal underneath to attack by the halogen gas in the chamber and causing halide dusts. Additionally, the arcing can cause impurities in the high pressure gas.
Prevention of arcing is discussed in U.S. Pat. No. 4,115,828 issued to Rowe et al. In the Rowe et al. Patent, an arc is detected in a continuous wave gas laser by sensing the voltage between electrodes in the laser. Upon sensing a voltage at which arcing occurs, a power supply to the electrodes is disconnected. Rowe et al. take about 5 to 10 milliseconds to shut down the power supply to the continuous wave gas laser. However, the technique employed by Rowe et al. is not fast enough for a pulse discharge laser because a pulse discharge laser requires response times of about 10 nanoseconds. Furthermore, the sensing circuit of Rowe et al. is subject to failure and difficult to adapt to different types of laser gasses.